1. Field of the Invention
The present invention relates to the application of a coating material to the outer surface of a pipe wherein neither the pipe nor the entire coating apparatus need be rotated to accomplish a coating around a complete circumferential area of the pipe.
2. Description of Related Art
Pipelines laid overland or under water are assembled from generally cylindrical sections of hollow pipe that are suitably joined together. A typical section that is used to fabricate an oil or gas pipeline has a length of approximately 20 metres and an outside diameter ranging from approximately 1 to 20 metres. A suitable joining process, such as welding, is used to join the pipe sections together. Each section of pipe is manufactured with an exterior coating that typically consists of an inner protective coating layer and an outer insulative layer. The protective layer, with a typical thickness of 1 mm, is formed by rotating the section of pipe whilst the material is applied to the pipe. A suitable composition is a fusion-bonded thermoplastic powder with an epoxy, polypropylene or polyethylene base that is applied to a pre-heated rotating section of pipe. The insulative layer, with a thickness generally on the order of 50 to 60 mm, is typically applied by an extrusion process. In order to join sections together, the insulative and protective coating layers must be cut or stripped back from each end of a section to expose the pipe material for the joining process. After the joining is completed, the exterior coating must be restored in the field to ensure integral coating of the pipeline. When a thermoplastic material is used, the coating material, in powdered form, is applied to the exterior of a pipe that has been preheated to achieve fusion of the material when it comes in contact with the pipe. For ferrous pipes, heating is generally accomplished by magnetic induction. Prior art processes and apparatus for accomplishing this task are disclosed in U.S. Pat. No. 4,595,607. An adhesive material, which can be a polypropylene-based composition, is normally applied over the protective coating by a similar process. Finally, the thicker insulative material is laid over the adhesive by an extrusion process.
Exterior protective coating of an entire pipe may be accomplished by an electrostatic process in which a pipe that has an induced charge on its surface is rotated over a coating material having an opposing charge.
A disadvantage of the prior art is that either the entire coating apparatus or the pipe must be rotated to achieve a full 360-degree coating of an area around the outer perimeter of the pipe.
Therefore, there exists the need for apparatus and method that can apply a 360-degree perimetrical band of coating material to the exterior surface of a pipe without rotating either the pipe or the coating apparatus.
An object of the present invention is to provide apparatus and method for applying a coating material around the complete perimeter of the exterior of a pipe without rotating the pipe or all components of the coating device. An outer stationary or stator element remains static whilst an inner rotor element is used to achieve a 360-degree perimetrical coating.
Another object of the present invention is to provide apparatus and method for applying a coating material around the complete perimeter of the exterior of a pipe without rotating the pipe or the coating device. The entire coating device remains stationary whilst a 360-degree perimetrical coating of the pipe is achieved.